1. Field of the Invention
The present invention relates to a crystalline optical fiber for infrared rays which has a core-crust structure and to a method of manufacturing the crystalline optical fiber.
2. Background of the Invention
A quartz glass fiber has been widely used for optical communication and has produced good results. There are two types of quartz glass fibers. One of them is the step index type. The other is the graded index type. The quartz glass fiber of the step index type has a double structure consisting of a core and a crust. This crust in a quartz fiber is usually called a cladding.
Although such quartz glass fibers have good properties, they can only be used for visible light or near infrared rays.
The light of a CO.sub.2 (carbon dioxide) laser has a wide range of use for industry and medical treatment. However, since the wave-length of the light is as long as 10.6 .mu.m, it cannot be transmitted with a low loss through any glass fiber developed so far.
Although some fibers have been developed for a CO.sub.2 laser, a relatively large loss is caused at the time of the transmission of the light of the laser. Therefore, the fibers are only used for communication at a short distance of several meters or less. However, the light of the CO.sub.2 laser is often used as power for material processing. Therefore, the fibers are useful enough for the light as power for such purposes.
A glass fiber and a crystalline fiber have been developed as optical fibers through which the light of a CO.sub.2 laser can be transmitted. The glass fiber is made of a chalcogenide glass, a fluoride glass or the like. The crystalline fiber is made of a metal halide crystal which can be of three broadly classified kinds as follows:
(1) Silver halide crystal AgBr, AgCl, AgI or a mixture thereof
(2) Thallium halide crystal TlBr, TlCl, TlI or a mixture thereof
(3) Alkali halide crystal CsI, CsBr or a mixture thereof
Other crystalline optical fibers of ZnSe, ZnS and so forth have also been developed.
One of methods of manufacturing a crystalline optical fiber is an extrusion method in which the manufacturing is started with a preform. Other methods include a pull-up method and a pull-down method in which the manufacturing is started with a molten material.
In the extrusion method, the preform of a single crystal is put in a container, softened by heat and extruded through a die to manufacture a thin fiber.
In the pull-up and the pull-down methods, the material is put in a crucible and is melted. In the pull-up method, a seed crystal is immersed in the molten material and gradually pulled up in the same manner as an ordinary growth of a crystal. In the pull-down method, a hole is previously provided in the bottom of the crucible so that a crystal is gradually pulled down through the hole.
Since the speed of the extrusion is high, the productivity of the extrusion method is higher than those of the pull-up and the pull-down methods.
The present invention relates particularly to an improvement in manufacturing a crytalline optical fiber in the extrusion method.
Each of the above-mentioned metal halide crystalline optical fibers is normally made of only a core with no crust. The core is inserted into a polymer tube when the core is used as the optical fiber. In that case, the air between the core and the tube acts as a crust. Since the core and the air differ from each other in refractive index, infrared rays are transmitted through the core while being totally reflected by the boundary between the core and the air. However, the air only acts as a very unstable crust.
The surface of the core, which plays an extremely important role in the transmission of light, is likely to be contaminated or damaged depending on the environment around the optical fiber or the its conditions. When the surface of the core is contaminated or a water drop clings to the surface, an intense energy absorption takes place because the contaminations and water have large absorption coefficient in the infrared region. Since the light of a CO.sub.2 laser is intense, heating results from the absorption to destroy the optical fiber. However, such a problem can be solved by providing the optical fiber with a crust which prevents the absorption.
In Japanese patent application (OPI) No. 132301/81 (the term "OPI" as used herein means an "unexamined published application") published on Oct. 26, 1981, it was proposed that the peripheral surface of the core of an optical fiber for infrared rays be coated with a metal film through evaporation in a vacuum so as to protect the core and to prevent the infrared rays from leaking out of the surface of the core. The film is made of gold with a thickness of l .mu.m. Although gold reflects infrared rays well enough to effectively confine them in the core, the optical fiber coated with the gold has disadvantages as described next. It is difficult to uniformly coat the peripheral surface of the core of circular cross section with the gold by evaporation. The gold prepared as the material for the coating film is significantly wasted. The thickness of the coating metal film cannot be made larger than about 1 .mu.m. For that reason, the film cannot physically protect the core or sufficiently confine the infrared rays in the core. Since gold is not transparent to infrared rays, the gold intensely absorbs them so that the gold is heated. Also for the same reason, the thickness of the film cannot be made large. Therefore, the crust on the core should be made of a material through which the infrared rays are well transmitted similarly to a quartz glass fiber.
Although a method of manufacturing a crystalline optical fiber comprising a core of a crystal for infrared rays and a crust has been proposed, the number of the manufacturing steps of the method is large and the method is complicated.
A method of manufacturing a core and a crust at a single time unlike the above-mentioned prior art was proposed in the Japanese patent application (OPI) No. 208506/82 published on Dec. 21, 1982. In this method, a solid solution crystal of TlBr and TlI is used. The preformed crystal is put in a die, heated and extruded through a thin nozzle to manufacture a thin fiber. The temperature of the crystal seems to play an important role in the method. The temperatures of the die and the nozzle and the speed of the extrusion are set at 300.degree. C., 220.degree. C. and 10 mm/min., respectively. As a result, the crystal grains of the peripheral portion of the fiber are made small and those of its central portion are made large. Since the central and the peripheral portions differ from each other in crystal grain diameter, the fiber has a double structure consisting of a core and a crust. The optical fiber having the core-crust structure is thus manufactured in the simple extrusion method. The lower the temperatures are, the smaller the diameter of each crystal grain is. The grain diameter is 20 .mu.m, 100 .mu.m, and 200 .mu.m, respectively when the temperature of the nozzle is 300.degree. C., 350.degree. C., and 400.degree. C. Since the temperature of the nozzle is lower than that of the die, the peripheral portion of the material being extruded through the nozzle is cooled more than the central portion thereof so that the crystal grains of the peripheral portion become small and those of the central portion become large. The phenomenon that the double structure of the optical fiber confines light therein is explained by the facts that the angle of divergence of the light is small and the transmission factor of the fiber is high. The transmission factor of an optical fiber made of the same material but having no core-crust structure is 90%, while that of the optical fiber having the core-crust structure is 92%. In the method, heaters are non-uniformly distributed in order to make the temperature of the nozzle lower than that of the die.